Optical gratings are useful in controlling the paths or properties of traveling light. Of various types of gratings, those formed in optical fibers are of particular interest as they are key components in modern telecommunication systems. Basically, optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. The optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the indices of refraction.
Optical gratings are important elements for selectively controlling specific wavelengths of light within optical communication systems. Such gratings include Bragg gratings, long period gratings and diffraction gratings. Such a grating typically comprises a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves. Reconfigurabilty would be highly useful in all types of gratings.
A typical Bragg grating comprises a length of optical fiber, including a plurality of perturbations in the index of refraction substantially equally spaced along the fiber length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the fundamental mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping optical signal channels, stabilization of semi-conductor lasers, reflection of fiber amplifier pump energy, and dispersion compensation.
Bragg gratings are conveniently fabricated by doping a fiber core with one or more dopants sensitive to ultraviolet light, e.g., germanium or phosphorous, and exposing the core at spatially periodic intervals to a high intensity ultraviolet light source, e.g., an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A difficulty with conventional Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects only light in a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However in many applications, such as wavelength division multiplexing (WDM, especially dense WDM), it is desirable to have a reconfigurable grating whose wavelength response can be controllably altered.
One attempt to make a tunable waveguide grating uses a piezoelectric element to strain the grating. See Quetel et al., 1996 Technical Digest Series, Conf. on Optical Fiber Communication, San Jose, Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120, paper No. WF6. The difficulty with this approach is that the strain produced by piezoelectric a actuation is relatively small, limiting the tuning range of the device. Moreover, it requires a continuous application of electrical power with relatively high voltage (approximately 100 volts).
U.S. patent application Ser. No. 08/791,081 filed by Jin et al. on Jan. 29, 1997, and U.S. patent application Ser. No. 09/020,206, filed Feb. 6, 1998, both describe magnetically tunable optical fiber gratings including devices that are wavelength tunable and latchable by magnetic force. The tensile force in the fiber grating introduced by attractive or repulsive force between magnetic components alters the spacing between perturbations and hence the grating wavelength.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.'.gtoreq.10 .lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5 .LAMBDA.' to 4/5 .LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the fiber core, long-period gratings remove light without reflection, as by converting it from a guided mode to a non-guided mode. (A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, the non-guided is a cladding mode). The spacing .LAMBDA.' of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p into a nonguided mode, thereby reducing in intensity a band of light centered about .lambda..sub.p. Alternatively, the spacing .LAMBDA.' can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly needed for equalizing amplifier gain at different wavelengths.
A shortcoming of conventional long-period gratings for amplifier gain equalization, however, is their limited ability to dynamically equalize gain. They filter only a fixed wavelength. Each long-period grating with a given periodicity (.LAMBDA.') selectively filters light in a narrow bandwidth centered around. .lambda..sub.p =(n.sub.g -n.sub.ng). .LAMBDA.', where n.sub.g and n.sub.ng are the effective indices of the core and the cladding modes, respectively. The value of n.sub.g is dependent on the core and cladding refractive index while n.sub.ng is dependent on core, cladding and air indices.
In the future, multi-wavelength communication systems will require dynamic reconfiguration and reallocation of wavelengths among the various nodes of a network depending on user requirements, e.g., with programmable add/drop elements. This reconfiguration will impact upon the gain of the optical amplifier. As the number of channels passing through the amplifier changes or the power changes, the amplifier will start showing deleterious peaks in its gain spectrum, requiring modification of the long-period grating used to flatten the amplifier. Modifying the long-period grating implies altering either the center wavelength of the transmission spectrum and/or the depth of the coupling.
Thus, there is a need for reconfigurable long-period gratings whose transmission spectra can be controlled as a function of the number of channels and power levels transmitted through an amplifier. It is desirable to have reconfigurable long-period gratings which, upon activation, can be made to dynamically filter other wavelengths (i.e., besides .lambda..sub.p). It is also desirable to be able to selectively filter a broad range of wavelengths. Further, reconfigurable long period gratings would be useful for suppressing amplifier spontaneous emission (ASE), and could also be used for filtering out undesirable remnant signals from communication channel Add/Drop operations.
Diffraction gratings typically comprise reflective surfaces containing a large number of parallel etched lines of substantially equal spacing. Light reflected from the grating at a given angle has different spectral content dependent on the spacing. The spacing in conventional diffraction gratings, and hence the spectral content, is generally fixed. Thus diffraction gratings are also of limited reconfigurability.
In view of the foregoing, it can be seen that there is a need for programmable optical gratings including Bragg gratings, long-period gratings and diffraction gratings whose spacing can be latchably reconfigured.